This invention relates to startup circuits and, more particularly, to bootstrap startup circuits that are used in power supplies to initialize the bias voltage to integrated circuits.
Generally, a bootstrap startup circuit provides an initial voltage across a capacitor prior to a power supply for the integrated circuit attaining its predetermined value. Further, the bootstrap startup circuit is turned off once the power supply attains its predetermined value.
Typically, a bootstrap startup circuit may include a single resistor or a power up transistor circuit. However, a disadvantage of the single resistor bootstrap startup circuit is that it draws a significant amount of current, while a disadvantage of the integrated power up transistor bootstrap startup circuit is that the bias voltage range is limited to a maximum of approximately 10-15 volts. This limited bias voltage range may become a problem when a wider bias voltage range is needed such as when primary-side control integrated circuits have an input voltage range of 10-30 volts.
Hence, there is a need for a bootstrap startup circuit that has a lower current drain and for providing a wide bias voltage range.